1. Field of the Invention
The present invention relates to a surface acoustic wave filter, and more particularly, to a surface acoustic wave filter device including a ladder type circuit.
2. Description of the Related Art
A conventional band-pass filter includes a ladder type circuit having a plurality of one terminal pair surface acoustic wave resonators that are connected in a ladder configuration (e.g., Japanese Unexamined Patent Application Publication No. 05-18338).
In this band-pass filter, a plurality of one terminal pair surface acoustic wave resonators are connected in series between an input terminal and an output terminal as series arm resonators to define series arms. In addition, a plurality of one terminal pair surface acoustic wave resonators are connected in parallel between the series arms and ground as parallel arm resonators to define parallel arms.
The band-pass filter including a ladder type circuit has reduced insertion loss and a wide pass band, and is commonly used as a band-pass filter in a portable telephone.
Japanese Unexamined Patent Application Publication No. 05-183380 discloses that an inductance component is connected in series to the above-described series arm resonator or parallel arm resonator, and this provides an increased bandwidth.
Also, Japanese Unexamined Patent Application Publication No. 10-93382 discloses a structure wherein an inductance is added to parallel arm resonators in a surface acoustic wave filter device having a ladder type circuit. FIG. 18 shows a circuit configuration of the surface acoustic wave filter device according to this prior art. In the surface acoustic wave filter device 501, series arm resonators S1 and S2 and parallel arm resonators P1 to P3 are connected to one another so as to define a ladder type circuit. Here, an inductance L is inserted between the parallel arm resonators P1 to P3 and the ground potential to thereby provide an increased pass band width and an increased attenuation value in the vicinity of a pass band.
On the other hand, Japanese Unexamined Patent Application Publication No. 04-65909 discloses a structure in which a surface acoustic wave filter element is connected to a package by a face down mounting process. Conventionally, in the package in which the surface acoustic wave filter element is accommodated, the electrodes of a package and the electrodes of the surface acoustic wave filter element are connected by bonding wires. In contrast, the surface acoustic wave filter device disclosed in this prior art has a reduced size by utilizing the face down mounting process. FIG. 19 shows a schematic sectional view of a package of a surface acoustic wave filter element provided using this face down mounting process.
In the surface acoustic wave filter device 601, a surface acoustic wave filter element 603 is provided in a package 602. The package 602 includes a base board 602a, a side wall 602b, and a cap 602c. 
On the base board 602a, a die-attach portion 602d is provided having a plurality of electrode pads electrically connected to the surface acoustic wave filter element 603. The surface acoustic wave filter element 603 includes a piezoelectric substrate 603a, and electrodes of a surface acoustic wave filter element are provided on the bottom surface of the piezoelectric substrate 603a. The electrodes provided on the bottom surface of the piezoelectric substrate 603a are electrically connected to the electrode pads in the die-attach portion 602d via bumps 604, and the surface acoustic wave filter element 603 is mechanically fixed to the die-attach portion 602d via the bumps 604.
In a face down mounting method for bonding, a surface including electrodes defining a surface acoustic wave filter element of the piezoelectric substrate is attached to a package by bumps. Since no bonding wires are required, a surface acoustic wave filter device having a reduced size is produced.
As disclosed in the Japanese Unexamined Patent Application Publications Nos. 05-183380 and 10-93382, when an inductance is added to a series arm resonator or a parallel arm resonator in a surface acoustic wave filter device having a ladder type circuit, the filter characteristics thereof are improved. Also, when connecting a surface acoustic wave filter element and the electrodes on a package by a bonding wire, the above-described inductance can be included using the bonding wire.
However, in the surface acoustic wave filter device 601, which is packaged by above-described face down mounting process, since no bonding wires are provided, an inductance component cannot be added to the surface acoustic wave filter device 601 using the bonding wire. A small inductance component may be provided by rooting electrodes connecting the external electrodes provided on the package to the die-attach portion, but a large inductance cannot be obtained by such rooting electrodes.
Therefore, in the surface acoustic wave filter device disclosed in the Japanese Unexamined Patent Application Publication No. 04-65909, it is difficult to achieve an increased pass band width and an increased attenuation value in the vicinity of a pass band, by the addition of inductance.
Japanese Unexamined Patent Application Publication No. 04-65909 describes that, by adding inductance components between each of the input/output signal terminals and the ground terminal in the package, an input/output impedance matching is achieved without using external elements. This description, however, applies to a surface acoustic wave filter having a structure which must achieve an input/output impedance matching outside the surface acoustic wave filter. Accordingly, in the case of a surface acoustic wave filter having a ladder circuit configuration which does not need to be matched, it is unnecessary to achieve an impedance matching between each of the input/output signal terminals and the ground terminal in such a package.
Also, in the Japanese Unexamined Patent Application Publication No. 04-65909, inductance components are provided in the die-attach portion. However, with this structure, an electromagnetic coupling occurs between the wiring and the die-attach portion on the piezoelectric substrate of the surface acoustic wave filter element, such that the filter characteristics thereof deteriorate. In addition, it is necessary to adjust the position and the number of bumps to fix and electrically connect the surface acoustic wave filter element and the package in order to provide an inductance component in the die-attach portion. However, since the position and the number of bumps do not necessarily achieve the sufficient electrical connections and mechanical bonding, the reliability of surface acoustic wave filter device deteriorates.